Go to the church where Tyler's gang is guarding the chemicals. Ask about what's going on: you will need a Charisma of 5 or more in order to progress the quest (having less than 5 only lets you give Tyler a casual greeting without consequences, keep talking to him and eventually he will tell you of the party, and you can progress). Tyler will reveal that they are throwing a party.

When you return to Lara, you will find that you cannot collect your reward from her. Instead, she will demand that you help her attack the gang.